Electric motor for a power tool

ABSTRACT

A power tool includes a housing, a battery connection portion supported by the housing, a drive mechanism configured to operate a working element, and a brushless DC motor positioned within the housing and connected to the drive mechanism. The motor includes a rotor, stator surrounding the motor, and an output shaft fixed to the rotor such that movement of the rotor is transmitted to the output shaft. The motor also includes a fan coupled to the output shaft to rotate with the output shaft and the rotor, and a brass bushing fixed to the output shaft to rotate with the rotor and the output shaft relative to the stator. The bushing includes a balancing feature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/399,129, filed Apr. 30, 2019, now U.S. Pat. No. 10,700,575, which is a continuation of U.S. application Ser. No. 15/676,214, filed Aug. 14, 2017, now U.S. Pat. No. 10,476,350, which is a continuation of U.S. application Ser. No. 14/072,023, filed Nov. 5, 2013, now abandoned, which claims priority to U.S. Provisional Patent Application No. 61/723,166, filed Nov. 6, 2012, the entire contents of each of which are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to power tools and, more particularly, to electric motors for use with power tools.

BACKGROUND OF THE INVENTION

Many power tools use electric motors, such as brushless DC motors, to drive the tools. A brushless DC motor typically includes a rotor and a stator. During operation, the rotor rotates at a relatively high speed relative to the stator. If the motor is unbalanced, the power tool may vibrate or shake during use.

SUMMARY OF THE INVENTION

In one embodiment, the invention provides a power tool including a housing, and a battery connection portion supported by the housing. The battery connection portion is configured to receive a battery pack. The power tool also includes a drive mechanism configured to operate a working element, and a brushless DC motor positioned within the housing and connected to the drive mechanism. The brushless DC motor is electrically coupled to the battery connection portion. The brushless DC motor includes a rotor having a body, a plurality of slots formed in the body and extending between a first face and a second face of the body, and a plurality of interior permanent magnets. Each interior permanent magnet is positioned in a respective one of the plurality of slots. The brushless DC motor also includes a stator surrounding the rotor. The stator has a plurality of electromagnetic coils and laminations surrounding the plurality of electromagnetic coils. An output shaft is fixed to the rotor such that movement of the rotor is transmitted to the output shaft. The brushless DC motor further includes a fan coupled to the output shaft to rotate with the output shaft and the rotor. A brass bushing is fixed to the output shaft to rotate with the rotor and the output shaft relative to the stator. The brass bushing has at least one hole of removed material on a surface of the brass bushing to help balance the motor. The brass bushing has a smaller outer diameter than an outer diameter of the body of the rotor.

In another embodiment, the invention provides a brushless DC motor for use with a power tool. The brushless DC motor includes a rotor having a body, a plurality of slots formed in the body and extending between a first face and a second face of the body, and a plurality of interior permanent magnets. Each interior permanent magnet is positioned in a respective one of the plurality of slots. The brushless DC motor also includes a stator surrounding the rotor. The stator has a plurality of electromagnetic coils and laminations surrounding the plurality of electromagnetic coils. An output shaft is fixed to the rotor such that movement of the rotor is transmitted to the output shaft. The brushless DC motor further includes a fan coupled to the output shaft to rotate with the output shaft and the rotor. A brass bushing is fixed to the output shaft to rotate with the rotor and the output shaft relative to the stator. The brass bushing has at least one hole of removed material on a surface of the brass bushing to help balance the motor. The brass bushing has a smaller outer diameter than an outer diameter of the body of the rotor.

Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a power tool with a portion of a housing removed to show a motor.

FIG. 2 is a perspective view of the motor shown in FIG. 1.

FIG. 3 is a perspective view of a motor including a bushing.

FIG. 4 is a perspective view of the motor and the bushing of FIG. 3 with a balancing feature formed on the bushing.

FIG. 5 is a side view of the motor and the bushing of FIG. 3.

FIG. 6 is a perspective view of a motor shaft before and after the shaft is overmolded.

FIG. 7 is a perspective view of a motor including the overmolded shaft of FIG. 6.

FIG. 8 is a perspective view of a motor including a bushing with slots.

FIG. 9 is an end view of the motor and the bushing of FIG. 8.

FIG. 10 is a perspective view of a motor including a fan.

FIG. 11 is a perspective view of the motor and the fan of FIG. 10 with a balancing feature formed on the fan.

FIG. 12 is a perspective view of the motor and the fan of FIG. 10 with a balancing feature formed on a bushing that supports the fan.

FIG. 13 is a perspective view of a motor including a fan with slots.

FIG. 14 is an end view of the motor and the fan of FIG. 13.

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

DETAILED DESCRIPTION

FIG. 1 illustrates a power tool 10 including an electric motor 14, such as a brushless DC motor. In the illustrated embodiment, the power tool 10 is a hammer drill. In other embodiments, the power tool 10 may be, for example, a reciprocating saw, a circular saw, a jigsaw, a drill, an impact driver, a screwdriver, a pipe cutter, a grinder, a sander, a caulk gun, a grease gun, or the like. In further embodiments, the power tool 10 may be another type of device that uses an electric motor, such as a vacuum, a paint sprayer, lawn and garden equipment, or the like.

The motor 14 is positioned in a housing 18 of the power tool 10 and connected to a drive mechanism 22. The illustrated power tool 10 also includes a battery connection portion 26 formed on the housing 18. The battery connection portion 26 receives a battery pack to electrically couple the battery pack to the motor 14 through a switch and other electronics. The battery pack may be, for example, a 9V, 12V, 18V, 28V, or 36V Li-ion or NiMH battery pack. When powered, the electric motor 14 drives the drive mechanism 22 to operate a working element (e.g., a drill bit) connected to a chuck 30 of the power tool 10.

FIG. 2 illustrates the electric motor 14 in more detail. The motor 14 includes a rotor 34, a stator 38, and a motor shaft 42. The rotor 34 includes four permanent magnets 46 positioned in a body 50, or stack, of the rotor 34. The body 50 may be a laminated steel structure or powdered steel. The illustrated magnets 46 are received in corresponding slots 54 formed in the body 50 and extend between a first face 50A and a second face 50B (FIG. 5) of the body 50. The slots 54 are arranged generally perpendicular to one another in a square pattern in the body 50. The illustrated stator 38 includes six electromagnetic coils 58 surrounding the rotor 34. The coils 58 are surrounded by laminations 60 of the stator 38. When powered, the electromagnetic coils 58 create magnetic fields that interact with the permanent magnets 46 in the rotor 34 to rotate the rotor 34 relative to the stator 38. In other embodiments, the motor 14 may include fewer or more permanent magnets 46 in the rotor 34 and/or electromagnetic coils 58 in the stator 38. Although the illustrated magnets 46 are positioned within the rotor body 50 such that the magnets 46 are interior permanent magnets (IPM), in other embodiments the magnets 46 may be glued or otherwise affixed onto an outer surface of the rotor body 50 such that the magnets 46 are surface permanent magnets (SPM).

The motor shaft 42, or output shaft, extends out of the rotor 34 to rotate with the rotor 34. The shaft 42 defines a longitudinal axis 68 about which the rotor 34 rotates. The shaft 42 is fixed to the rotor 34 such that movement of the rotor 34 is transmitted to the shaft 42. In some embodiments, the shaft 42 may be fixed to the rotor 34 using suitable securing means, such as splines, knurls, press-fitting, adhesives, or the like. In other embodiments, a portion of the shaft 42 may be non-cylindrical to rotationally fix the shaft 42 to the rotor 34.

A first bearing 70 is positioned on a first end of the shaft 42 to support the shaft 42, and thereby the motor 14, within the housing 18 of the power tool 10. A fan or impeller 74 is coupled to a second end of the shaft 42 to rotate with the shaft 42 and the rotor 34. The fan 74 creates an airflow around the motor 14 to cool the motor 14 during operation of the power tool 10. A second bearing 78 (FIG. 5) is coupled to the second end of the shaft 42 adjacent the fan 74 to help support the motor 14 in the housing 18 of the power tool 10.

FIG. 3 illustrates a similar motor 114 including the rotor 34, the stator (not shown), the motor shaft 42, and the fan 74. In this embodiment, the motor 114 includes a balancing member 118 comprising a bushing 122. The illustrated bushing 122 is generally cylindrical and composed of a non-magnetic, high density material such as brass. The bushing 122 is coupled to the shaft 42 between the first face 50A of the rotor body 50 and the bearing 70 (FIG. 5). The bushing 122 is fixed to the shaft 42 to rotate with the rotor 34 and the shaft 42 relative to the stator. In some embodiments, the bushing 122 may be secured to the shaft 42 by splines, knurls, press-fitting, or the like.

When the bushing 122 is assembled on the shaft 42, the bushing 122 rotates with the rotor 34 to help balance the motor 114. If the motor 114 is still imbalanced, a balancing feature 126 (FIG. 4) is formed on the bushing 122 to correct the imbalance within a predetermined specification. As shown in FIG. 4, the illustrated balancing feature 126 is machined in the bushing 122 by drilling a hole into an outer circumferential surface 130 of the bushing 122. In other embodiments, other balancing features may be formed on the bushing 122 by cutting, shaving, or otherwise removing material from the bushing 122. Additionally or alternatively, multiple balancing features may be formed on the bushing 122 to balance the motor 114 within the predetermined specification.

As shown in FIG. 5, the bushing 122 also includes a flange or step 134. The step 134 has a smaller diameter than the remainder of the bushing 122 and extends axially from the bushing 122. The step 134 engages the bearing 70 to help support the bearing 70 on the shaft 42. In particular, the step 134 supports an inner face of the bearing 70 to prevent the bearing 70 from moving along the shaft 42 toward the rotor body 50 if the power tool is dropped or during high vibration tool use. In some embodiments, the step 134 may be omitted.

FIGS. 6 and 7 illustrate another motor 214 including the rotor 34, the stator (not shown), the motor shaft 42, and the fan 74. In this embodiment, the motor 214 includes a balancing member 218 comprising an overmold 222 on the shaft 42. FIG. 6 illustrates the shaft 42 before and after the overmold 222 is applied to the shaft 42. In the illustrated embodiment, the overmold 222 is a thermoset plastic or thermoplastic material molded over a knurled surface 226 of the shaft 42. In other embodiments, other suitable materials may also or alternatively be molded over the shaft 42. In still other embodiments, the overmold 222 could be molded over the shaft 42, the magnets 46, and the rotor body 50 or over the shaft 42 and the rotor body 50.

The illustrated overmold 222 includes a radially-extending flange 230 having a larger diameter than the remainder of the overmold 222. The flange 230 is formed near, but spaced from an end of the overmold 222 adjacent a first end of the shaft 42. The flange 230 provides a hard stop for the rotor body 50. In some embodiments, the flange 230 may also provide a hard stop for the bearing (not shown) positioned on the first end of the shaft 42. In such embodiments, the bearing may be positioned on a smaller diameter section 234 of the overmold 222. In other embodiments, the smaller diameter section 234 of the overmold 222 may abut the bearing to provide the hard stop.

Referring to FIG. 7, the rotor body 50 is pressed onto the overmold 222 to fix the body 50 and the shaft 42 together. The first face 50A of the body engages an inner face 238 (FIG. 6) of the flange 230 to inhibit axial movement of the body 50 along the overmold 222. In addition, the remainder of the overmold 222 provides a press surface for laminations of the rotor body 50.

When the overmold 222 is formed on the shaft 42, the overmold 222 helps balance the motor 214. In some embodiments, one or more balancing features may be formed on the overmold 222 (particularly in the radially-extending flange 230 of the overmold 222) to help correct any imbalances of the motor 214. For example, material may be cut, drilled, shaved or otherwise removed from the overmold 222 to balance the motor 214 within the predetermined specification.

In some embodiments, the balancing member 218 may also include a balance washer or bushing coupled to the overmold 222. The balance washer may be insert molded into the overmold 222 or may be assembled onto the overmold 222 (e.g., over the smaller diameter section 234 of the overmold 222 adjacent the first end of the shaft 42). In further embodiments, one or more balancing features may also or alternatively be formed in the washer to help balance the motor 214.

FIG. 8 illustrates another motor 314 including the rotor 34, the stator (not shown), the motor shaft 42, and the fan 74. In this embodiment, the motor 314 includes a balancing member 318 comprising a slotted bushing 322. The illustrated bushing 322 is generally cylindrical and composed of a non-magnetic material such as brass or plastic. The bushing 322 is coupled to the shaft 42 between the first face 50A of the rotor body 50 and the bearing (not shown). The bushing 322 is fixed to the shaft 42 to rotate with the rotor 34 and the shaft 42 relative to the stator. In some embodiments, the bushing 322 may be secured to the shaft 42 by splines, knurls, press-fitting, or the like.

In the illustrated embodiment, the bushing 322 includes wedge-shaped openings or slots 326 extending through the bushing 322. The illustrated bushing 322 includes fourteen slots 326 formed in a face of the bushing 322 and arranged circumferentially about the longitudinal axis 68. In other embodiments, the bushing 322 may include fewer or more slots 326. The slots 326 are separated by radially-extending sidewalls 330 such that each slot 326 is discrete from adjacent slots 326.

As shown in FIG. 9, a wedge-shaped insert 334 of balancing material is positioned within one of the slots 326 of the bushing 322. In some embodiments, the balancing material is a putty or soft epoxy that hardens and sets in the slot 326. In other embodiments, the insert 334 may be a separate piece that is shaped to generally match the shape of the slots 326 and is held in the corresponding slot 326 by press-fitting and/or adhesives. When positioned in the slot 326, the insert 334 is captured between the first face 50A of the rotor 34 and the bearing to inhibit the insert 334 from sliding out of the bushing 322. The bushing 322 and the insert 334 help correct imbalances in the motor 314 when the rotor 34 rotates relative to the stator. Although only one insert 334 is shown in one slot 326 of the bushing 322, it should be readily apparent that multiple inserts 334 may be positioned in multiple slots 326 to help balance the motor 314 within the predetermined specification.

In other embodiments, the slots 326 may be formed in an overmold covering the motor shaft 42, such as the overmold 222 shown in FIGS. 6 and 7. In such embodiments, the inserts 334 of balancing material may be positioned directly in the overmold, rather than in the bushing 322. The bushing 322 may therefore be omitted or may be used in combination with the overmold.

FIG. 10 illustrates another motor 414 including the rotor 34, the stator (not shown), the motor shaft 42, and the fan 74. In this embodiment, the motor 414 includes a balancing member 418 comprising the fan 74. As discussed above, the fan 74 rotates with the rotor 34 and the shaft 42 to create an airflow for cooling the motor 414 during operation. Blades or fins 422 extend from a first face 426 of the fan 74 to generate the airflow. In the illustrated embodiment, the fan 74 is seated on a bushing 430 that surrounds a portion of the shaft 42 adjacent the second face 50B of the rotor body 50. In some embodiments, the bushing 430 may be part of an overmold formed on the shaft 42, such as the overmold 222 shown in FIGS. 6 and 7.

As shown in FIG. 11, a balancing feature 434 is formed on a second face 438 of the fan 74 opposite the fins 422. The balancing feature 434 helps balance the motor 414 within the predetermined specification. In the illustrated embodiment, the balancing feature 434 is machined in the fan 74 by cutting material off of the second face 438 of the fan 74. In other embodiments, other balancing features may be formed on the fan 74 by drilling, shaving, or otherwise removing material from the fan 74. Additionally or alternatively, multiple balancing features 434 may be formed on the fan 74 to balance the motor 414 within the predetermined specification.

As shown in FIG. 12, a balancing feature 442 is formed on an outer face 446 of the bushing 430. The balancing feature 442 helps balance the motor 414 within the predetermined specification. In the illustrated embodiment, the balancing feature 442 is machined in the bushing 430 by drilling a hole into the outer face 446 of the bushing 430. In other embodiments, other balancing features may be formed on the bushing 430 by cutting, shaving, or otherwise removing material from the bushing 430. Additionally or alternatively, multiple balancing features 442 may be formed on the bushing 430 to balance the motor 414 within the predetermined specification. In further embodiments, balancing features may be formed on both the fan 74 and the bushing 430 to help balance the motor 414.

FIG. 13 illustrates another motor 514 including the rotor 34, the stator (not shown), and the motor shaft 42. In this embodiment, the motor 514 includes a balancing member 518 comprising a slotted fan 522. The illustrated fan 522 is similar to the fan 74 discussed above, but also includes wedge-shaped openings or slots 526. The slots 526 are formed in a face 530 of the fan 522 opposite from fins 534 of the fan 522. In the illustrated embodiment, the slots 526 are formed in an inner radial portion of the face 530. In other embodiments, the slots 526 may be formed in an outer radial portion of the face 530. The illustrated fan 522 includes fourteen slots formed in the face 530 of the fan 522 and arranged circumferentially about the longitudinal axis 68. In other embodiments, the fan 522 may include fewer or more slots 526. The slots 526 are separated by radially-extending sidewalls 538 such that each slot 526 is discrete from adjacent slots 526.

As shown in FIG. 14, a wedge-shaped insert 542 of balancing material is positioned within one of the slots 526 of the fan 522. In some embodiments, the balancing material is a putty or soft epoxy that hardens and sets in the slot 526. In other embodiments, the insert 542 may be a separate piece that is shaped to generally match the shape of the slots 526 and is held in the corresponding slot 526 by press-fitting and/or adhesives. The insert 542 helps correct imbalances in the motor 514 when the rotor 34 rotates relative to the stator. Although only one insert 542 is shown in one slot 526 of the fan 522, it should be readily apparent that multiple inserts 542 may be positioned in multiple slots 526 to help balance the motor 514 within the predetermined specification.

The balancing members 118, 218, 318, 418, 518 discussed above help balance an electric motor in a power tool, or other device, within a predetermined specification to reduce vibration of the motor during use. In particular, the balancing members 118, 218, 318, 418, 518 balance masses of the corresponding rotors about the longitudinal axis of the motors (e.g., about the longitudinal axis 68) to reduce vibrations of the motors compared to motors without balancing members. Although each of the balancing members 118, 218, 318, 418, 518 has been described separately, a single motor may include a combination of two or more of the balancing members 118, 218, 318, 418, 518 to achieve the desired balance.

Although the invention has been described with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of one or more independent aspects of the invention as described. 

What is claimed is:
 1. A power tool comprising: a housing; a battery connection portion supported by the housing, the battery connection portion configured to receive a battery pack; a drive mechanism configured to operate a working element; and a brushless DC motor positioned within the housing and connected to the drive mechanism, the brushless DC motor electrically coupled to the battery connection portion and including a rotor having a body, a plurality of slots formed in the body and extending between a first face and a second face of the body, and a plurality of interior permanent magnets, each interior permanent magnet positioned in a respective one of the plurality of slots, a stator surrounding the rotor, the stator having a plurality of electromagnetic coils and laminations surrounding the plurality of electromagnetic coils, an output shaft fixed to the rotor such that movement of the rotor is transmitted to the output shaft, a fan coupled to the output shaft to rotate with the output shaft and the rotor, and a brass bushing fixed to the output shaft to rotate with the rotor and the output shaft relative to the stator, the brass bushing having at least one hole of removed material on a surface of the brass bushing to help balance the motor, wherein the brass bushing has a smaller outer diameter than an outer diameter of the body of the rotor.
 2. The power tool of claim 1, wherein the brushless DC motor also includes a bearing positioned on the output shaft, wherein the brass bushing is coupled to the output shaft between the first face of the body of the rotor and the bearing.
 3. The power tool of claim 2, wherein the bearing is a first bearing positioned on a first end of the output shaft, and wherein the brushless DC motor further includes a second bearing positioned on a second end of the output shaft.
 4. The power tool of claim 3, wherein the fan is adjacent the second bearing.
 5. The power tool of claim 1, wherein the plurality of slots includes four slots, and wherein the plurality of interior permanent magnets includes four interior permanent magnets.
 6. The power tool of claim 5, wherein the four slots are arranged generally perpendicular to another in a square pattern.
 7. The power tool of claim 1, wherein the laminations of the stator define a plurality of grooves extending axially along an outer periphery of the stator.
 8. The power tool of claim 1, wherein the body of the rotor is formed as a laminated steel structure.
 9. The power tool of claim 1, wherein the stator includes bosses that receive fasteners for mounting the brushless DC motor to the housing.
 10. The power tool of claim 1, wherein the power tool is one selected from the group consisting of a hammer drill, a reciprocating saw, a circular saw, a jigsaw, a drill, an impact driver, a screwdriver, a pipe cutter, a grinder, a sander, a caulk gun, and a grease gun.
 11. The power tool of claim 1, wherein the power tool is a saw.
 12. The power tool of claim 1, wherein the battery connection portion is configured to receive a Li-ion battery pack.
 13. A brushless DC motor for use with a power tool, the brushless DC motor comprising: a rotor having a body, a plurality of slots formed in the body and extending between a first face and a second face of the body, and a plurality of interior permanent magnets, each interior permanent magnet positioned in a respective one of the plurality of slots; a stator surrounding the rotor, the stator having a plurality of electromagnetic coils and laminations surrounding the plurality of electromagnetic coils; an output shaft fixed to the rotor such that movement of the rotor is transmitted to the output shaft; a fan coupled to the output shaft to rotate with the output shaft and the rotor; and a brass bushing fixed to the output shaft to rotate with the rotor and the output shaft relative to the stator, the brass bushing having at least one hole of removed material on a surface of the brass bushing to help balance the motor, wherein the brass bushing has a smaller outer diameter than an outer diameter of the body of the rotor.
 14. The brushless DC motor of claim 13, further comprising a bearing positioned on the output shaft, wherein the brass bushing is coupled to the output shaft between the first face of the body of the rotor and the bearing.
 15. The brushless DC motor of claim 14, wherein the bearing is a first bearing positioned on a first end of the output shaft, and wherein the brushless DC motor further comprises a second bearing positioned on a second end of the output shaft.
 16. The brushless DC motor of claim 15, wherein the fan is adjacent the second bearing.
 17. The brushless DC motor of claim 13, wherein the plurality of slots includes four slots, and wherein the plurality of interior permanent magnets includes four interior permanent magnets.
 18. The brushless DC motor of claim 13, wherein the laminations of the stator define a plurality of grooves extending axially along an outer periphery of the stator.
 19. The brushless DC motor of claim 13, wherein the body of the rotor is formed as a laminated steel structure.
 20. The brushless DC motor of claim 13, wherein the stator includes bosses that receive fasteners for mounting the brushless DC motor to a housing of the power tool. 